In recent years, electric cars have attracted attention in view of the environmental problems. An electric car has a rotary electric machine (motor) as a power source and a smoothing transformer (reactor), for an inverter circuit output, and improvement has been demanded for the efficiency of such parts. Magnetic cores used for the parts described above are required to have low iron loss and high magnetic flux density, as well as that magnetic properties thereof are not lowered in a region from low frequency to high frequency.
The iron loss includes an eddy current loss (We) greatly concerned with the specific resistance of the magnetic core and a hysteresis loss undergoing the effect of strains in the iron powder caused from the manufacturing process of the iron powder and the subsequent process hysteresis. The iron loss (W) can be shown by the sum for the eddy current loss (We) and the hysteresis loss (Wh) as shown in the following formula. In the formula, f represents a frequency, Bm represents an exciting field magnetic flux density, ρ represents a specific resistance, t represents the thickness of a material, and k1 and k2 each represents a coefficient.W=We+Wh=(k1Bm2t2/ρ)f2+k2Bm1.6f 
As apparent from the formula, since the eddy current loss (We) increases in proportion with the square of the frequency f, suppression for the eddy current loss is essential for not lowering the magnetic property, particularly, at high frequency. For suppressing the generation of the eddy current in the compacted powder magnetic core, it is necessary to optimize the size of the magnetic powder used, form an insulation layer on the surface of individual magnetic powder, and use a compacted powder magnetic core formed by compacting molding using such magnetic powder.
In the compacted powder magnetic core, in a case where the insulation is insufficient, the specific resistance ρ lowers to increase the eddy current loss. On the other hand, in a case where the thickness of the layer or film is increased for improving the insulation property, the ratio of the volume of the soft magnetic powder in the magnetic core is decreased to lower the magnetic flux density. For improving the magnetic flux density, in a case where the density of the soft magnetic powder is increased by conducting compacting molding of the soft magnetic powder at a high pressure, strain of the soft magnetic powder during molding is inevitable to increase the hysteresis loss (Wh) to result in a difficulty for the suppression of the iron loss (W). Since the eddy current loss (We) is low, particularly, in a low frequency region, the effect of the hysteresis loss (Wh) in the iron loss (W) increases.
In view of the problems described above, it has been proposed a method of forming an insulative layer on the surface of soft magnetic powdery particles by mixing a soft magnetic powder and an insulative particle such as of titania, silica or alumina (for example, refer to JP-A No. 2003-332116 (in claims)). Further, it has been proposed a method of manufacturing a compacted powder magnetic core by forming an insulation layer such as an oxide film or phosphate salt film on the surface of an Fe—Si type soft magnetic powder by way of compacting molding (for example, refer to JP-A No. 2004-288983 (in claims)). Further, it has been proposed a method of obtaining a high resistance by coating a phosphate salt in a liquid form and fixing the same by a post treatment (for example, refer to JP No. 3475041 (in claim 1)).
However, any of the methods described above involves a drawback that the occupation ratio of the iron powder particle is lowered by a binder and the magnetic flux density is not increased. Further, for removing strains, while the compacted powder magnetic core is applied with annealing at a temperature of 600° C. or higher after molding, in a case of forming an insulation layer by using phosphorous or oxygen, film forming elements diffuse in iron or form compounds with iron during annealing to possibly result in destruction of the insulation layer or degradation of the magnetic property.
In view of the above, a method of forming a fluoride film on the surface of the soft magnetic powder has been proposed (for example, refer to JP-A No. 2006-41203 (abstract).
Rare earth metal fluorides or alkaline earth metal fluorides are excellent in the heat resistance, less reactive with iron, and are extremely suitable as an insulation layer material for the compacted powder magnetic core. The method of forming the fluoride film on the surface of the soft magnetic powder also has an advantage capable of obtaining high resistance without a binder. Further, by the use of a gas atomized powder or an indefinite powder for the soft magnetic powder, improvement for the specific resistance and the high magnetic flux density can be attained.
However, in a case of applying a compacted powder magnetic core formed with a film of a rare earth metal fluoride or an alkaline earth metal fluoride to various kinds of motor yokes, it has been found that no predetermined performance can be provided. Specifically, compacted powder magnetic cores sometimes resulted in unexpected lowering of the efficiency when operated at a high number of rotation or applied to large-sized electric motors.
This is because conditions for operating the compacted powder magnetic core at a good efficiency change depending on the shape and the number of rotation of rotary machines to be applied, or the operation frequency and the shape of reactors to be applied. The specific resistance ρ(Ω·m), the thickness t (m), the magnetic material permeability μ, frequency f (Hz), and the thickness S (m) undergoing the skin effect are in a relation shown by the following formula and the magnetic body is not operated at a thickness exceeding the thickness S (m) undergoing the skin effect, which is considered as a loss.S=√(2p/2πfμ)
For example, at an operation frequency of the rotary machine of 400 Hz, a magnetic permeability of 500, and a thickness as 5 mm, ρ is 2×10−3 Ω·cm and a higher specific resistance is necessary.
The compacted powder magnetic core manufactured by the method described in JP-A No. 2006-41203 cannot yet be considered sufficient in view of the specific resistance.    Patent document No. 1: Japanese patent laid-open 2003-332116    Patent document No. 2: Japanese patent laid-open 2004-288983    Patent document No. 3: Japanese patent No. 3475041    Patent document No. 1: Japanese patent laid-open 2006-41203